71 
conglomerate overlying the upper melaphyr should not be cor- 
related with that forming the adjacent ledges of Conglomerate 
Plateau, but rather with that outeropping on Long Beach 
Rock and underlying the melaphyr series of Atlantic Hill. 
Conglomerate Plateau.—This broad mass of conglomerate, 
the largest in the Nantasket district, is of the normal type, well 
rounded pebbles of felsite and granite predominating, although 
pebbles of porphyrite and compact melaphyr are also common. 
The texture varies from moderately coarse conglomerate to 
sandstone ; but the bedding can be made out at only a few points 
as on the north side of Spring Valley Road. The dip seems, 
however, to be always gentle (5°—10°) to the south-southeast, 
diminishing toward the granite. The prominent ledges ex- 
hibit some well developed joint-planes ; and the plateau is un- 
doubtedly bounded on all sides, and not alone on the north, by 
fault-lines. It rises abruptly from the marsh on the west ; Hull 
Street and Spring Valley Road traverse it in narrow defiles ; 
while the straight and solid wall of conglomerate 15 to 80 feet 
high in which it terminates on the south is an exceptionally typ- 
ical fault scarp. Facts will be presented later tending to show 
that the downthrow in the case of all of these faults is on the side 
away from the plateau, and hence that this conglomerate is older 
than and normally underlies the surrounding formations ; but, as 
already indicated, the displacement is, much greater on the north 
side than in any other direction. Of the thickness of the con- 
glomerate it is impossible to speak with certainty, since neither 
the base nor, probably, the original summit of the bed are ex- 
posed. It can not be less than the height of the plateau, about 
fifty feet, and may exceed one hundred feet, this being, prob- 
ably, the thickest of all the Nantasket beds. 
Round Hill.—This hill, which, as the map indicates, is named 
for its curvetl outline, is a solid mass of conglomerate rising 
very abruptly from the eastern marsh on the north and west to 
to a height of fifty feet and sloping gently down to the south- 
